This feature can be used to hook into the logging subsystem and display warnings
which would be sent to Log4J or the java.util.logging package.

We'll be the first to admit this class is something of an afterthought and a slightly
unusual fit with the rest of the API, due mainly to the difficulty in associating log
messages (which are static, can emanate from any thread, and may not have a PDF object
associated with them) to a particular JComponent for the notification.
Consequently this class is referenced from various points in the API, particularly
where background threads are started to process the PDF.

This class will pop up a translucent window in the bottom-right corner of the
viewer which contains the warning messages, and each message will expire after 5 seconds.
We anticipate this display won't be to everyone's taste, in which case override this
class and implement your own warningEvent method. For example, to log messages
to the JavaScript console instead:

register

Register a Thread with this object. Any warning messages which
can be associated with this Thread will be sent to the specified
JComponent. This method is called by the various background threads
run in the viewer, e.g. thumbnail painting.